A conventional linear-motion engine, such as a Stirling engine, includes a displacement piston coupled to a power piston. The displacement piston moves as a result of applied thermal energy, which, in turn, causes the power piston to move. Electrical power is generated due to the motion of the power piston. More specifically, the power piston is coupled to an alternator, which converts the power piston's motion into electrical power.
The alternator includes two main parts, a stationary part referred to as the stator and a moving part referred to as the armature. Typically, the stator is made from a slotted iron core with copper wire windings wound inside the slots. The armature usually includes a set of permanent magnets mounted on a rigid body, preferably formed from a nonmagnetic material, such as aluminum that is coupled to the power piston.
The amplitude and polarity of magnetic flux created by the armature's permanent magnets changes when the power piston moves the armature. A voltage is produced across the stator's windings as a result of the change in magnetic flux. The voltage produced is referred to as the electromotive force, or emf, and it is defined as follows:emf=N×dφflex/dtwhere φflux=Bmag×W×Xp, and
emf=electromotive force
N=the number of turns in the stator's windings
dφflux/dt=the rate of magnetic flux change
W=the width of the armature's permanent magnets
Bmag=the magnetic field strength of the armature's permanent magnets
Xp=the power piston's position amplitude
From the above expression, if φflux is a sinusoidal waveform the emf can be expressed asemf=Bmag×W×Xp×Fwhere
F=the piston frequency
Thus, a direct relationship exists between the position amplitude of the power piston and the emf of the alternator. For a constant piston frequency, the greater the emf, the greater the range of movement of the power piston. The smaller the emf, the smaller the range of motion of the power piston. Because of this relationship, the power piston's position can be controlled by controlling the amplitude of the emf. This is particularly desired since Stirling engines are typically optimized to run at a constant piston frequency.
Under load, the voltage across the terminals of the alternator differs both in magnitude and phase from the emf. Primarily, the difference between the voltage across the terminals of the alternator and the emf is due to the impedance, i.e., inductance and resistance, of the stator windings. Typically, the inductive term is the dominant term of the stator winding impedance. In order to control the piston position amplitude, the emf must be directly accessible. Typically, a tuning capacitor is placed in series with the alternator in order to null out the effect of the inductance of the stator windings. Use of the tuning capacitor to facilitate access to the emf, allows for the use of a variety of loads with voltage limiting capability to limit the piston amplitude.
There are several disadvantages associated with coupling a tuning capacitor between the alternator and the load. First, the capacitance of the tuning capacitor may need to be adjusted due to changes in the impedance of the load over time. Otherwise, the efficiency of the linear-motion engine may be compromised. Second, tuning capacitors are costly since they are low tolerance components. Third, the tuning capacitors may require significant space, depending on the electrical characteristics of the load. Finally, a tuning capacitor does not compensate for the resistance of the stator windings, which contributes to an uncontrolled growth in amplitude, which may result in piston over-stroke.
Accordingly, there is a need for a low-cost apparatus used to control the movement of a power piston in a linear-motion engine so as to efficiently and effectively drive a load without requiring the use of a tuning capacitor. The present invention satisfies this need.